This invention relates to a high speed image drawing method permitting the drawing of an image including curves in an image output region at a high speed and with a visually more natural appearance, and to an apparatus for realizing the method.
In the case where a raster scan type CRT display is used as a graphic terminal for a computer system, it is usual to draw an image on the screen by illuminating selectively desired pixels on the screen, in the case of an image output region composed of a plurality of pixels arranged in a matrix form. Further, a plasma display or a display consisting of light emitting diodes arranged in a matrix form is a typical image output apparatus constituted by pixels arranged in a matrix form, for each of which a light emitting element operates as a pixel.
When a desired image is drawn on such an image output apparatus composed of pixels arranged in a matrix form, any image drawn in the image output region cannot be represented by a smooth curve. For example, when the image to be drawn is an ellipse 1 as indicated in FIG. 1A, the image 3 actually drawn in the image output region composed of pixels arranged in a matrix is an approximate image composed of a set of segments, as indicated by 2 in FIG. 1B.
Heretofore, in the case where an image was displayed in an image output region composed of pixels arranged in a matrix form, since pixels to be illuminated were selected by calculating for each of the pixels which was the closest pixel in the image output region to the image to be drawn, the time necessary for the drawing was long. For example, as seen in FIG. 2 suppose that it is to be determined which pixel after a pixel P.sub.o is to be illuminated, when an image S represented by a function F(x, y)=0 is being drawn in the direction indicated by the arrow. According to one of the methods proposed heretofore, at first 8 pixels P.sub.1, P.sub.2, . . . , P.sub.8 adjacent to the pixel P.sub.o in the up and down directions, and right and left directions are selected as candidates for the pixel which is to be illuminated next. Then the pixels P.sub.4, . . . , P.sub.8 are excluded, by considering the inclination of the image S to be drawn and the direction of the advance of the drawing. After that, for each of the remaining 3 candidate pixels P.sub.1, P.sub.2 and P.sub.3, the distance therefrom to the image S to be drawn, which is represented by the function F(x, y)=0, is calculated and the pixel having the smallest distance is selected as the pixel, which is to be illuminated next. Since a number of calculations should be repeated for each of the pixels in this way, a long time is necessary for producing the drawing in this way.
Further, according to the prior art method described above, since the pixel to be illuminated was determined only on the basis of calculation results, sometimes visually unnatural parts appeared in the image drawn in the image output region or on the display screen. For example, according to the prior art method, it happened that the pixels indicated in FIG. 3A were selected so as to be illuminated for the image to be drawn. However, when an observer looks at the image screen, on which such an image is drawn, since the part indicated by Q in FIG. 3A has a higher pixel density than the others, it looks like a protuberance-like lump attached on a curve so that the observer finds it ugly. It was found that if the pixels to be illuminated are selected as indicated in FIG. 3B for the same image as that for FIG. 3A, the observer finds it more natural.